Field of the Invention
Two color printing machines are predominantly used in plastic bag printing presses of the type shown in FIG. 3. As shown in the Figure, the prior art plastic bag printing machine 1 comprises a plastic bag wheel 11 which passes through a first printing means 12 to a second printing means 13 for application of the second color. The plastic bags are rolled to a drying means 18 and then collected on another plastic bag wheel 19. The first and second printing means 12 and 13 respectively comprises a rubber cylinder 14, an intermediate cylinder 15, a plate cylinder 16, and a pressing cylinder 17. In printing, the above plate cylinder 16 comes in contact with a set of inking rollers, not shown. For adjustment of the speed of the plate cylinder relative to the pressing cylinder, an adjustment device may be provided on one side of the plate cylinder 16.
A prior art adjustment device for use on the printing press of FIG. 3 is shown in FIG. 4. A fixing ring 7 is fitted on a main shaft 4 and a gear ring 5. The gear ring 5 is provided with an internal helical thread to engage with another helical thread provided on a sleeve 6 which is secured on the driving shaft 4. The relative motion between the gear ring 5 and the sleeve 6 will cause the driving shaft 4 to slide axially. The driving shaft 4 is provided with a first bevel gear 8 on one end and the pressing roller 17 at the other end (not shown). A second bevel gear 81 which is provided at one end of a driven shaft 333 engages with the first bevel gear 8. The driven shaft 333 is secured through bearings in a mounting member which is located in a housing 300. When the gear ring 5 and the sleeve 6 rotate, one relative to the other, such causes the driving shaft 4 to slide, causing a longitudinal shift x between the first bevel gear 8 and the second bevel gear 81. This also results in an axial shift y between the bevel gears. The axial shift between the first bevel gear 8 and the second bevel gear 81 causes the contacting position of pressing roller 17 and plate roller 16 to correspondingly shift to a new contacting position. Thus, the position on the plate roller 16 for registration of one color on the plastic bag will shift to a desired position.
However, there are many problems associated with the prior art system. First, the distance between the upper printing means and the lower printing means is limited by the size of the bevel gears. If a greater adjustment of distance is required, the interruption of printing is the only way to accomplish this. Second, the small contacting volume of the bevel gears will frequently cause their disengagement. Third, the speed of operation in printing is slow.
One object of this invention is to provide an adjustment apparatus for multi-color plastic bag printers which overcomes the problems of the prior art.
Other objects, merits a fuller understanding of the present invention will be obtained by those having ordinary skill in the art when the following detailed description of the preferred embodiment of the invention is read in conjunction with accompanying drawings wherein like numerals refer to like or similar parts and in which: